Optical fiber communication systems advantageously allow for efficient transmission of a large volume of information. There are two reasons that optical fiber communication systems allow for efficient information transfer. The first reason is that the low loss characteristic of optical fibers as transfer media causes little attenuation loss of signals during the transfer. Consequently, devices such as repeaters or generators necessary for long-distance transfer can advantageously be eliminated. The second reason is that hardware necessary for transmission/reception of optical signals can be cut down by time division multiplexing. Consequently, the cost necessary for transfer of the same volume can advantageously be reduced. The second reason is particularly useful for increased communication volume due to extensive use of the Internet. For example, some commercial optical fiber communication systems transfer 10 Gb/s per wavelength. Recently, some optical fiber communication systems have started to transfer 40 Gb/s per wavelength.
In order to deal with a larger volume of information communication in the future, ultrahigh-speed signal light transfer technology capable of multiplexing more signals is demanded. In response to such demand, intensive development and research has been conducted on technologies relating to signal transfer in a speed class of 100 Gb/s (approximately 100 Gb/s) or higher.
The NRZ (non-return-to-zero) code of which signals are easy to generate and detect is extensively used in optical fiber communication systems up to 10 Gb/s per wavelength. In an NRZ system, binary signals are encoded to optical ON and OFF for transfer.
Ultrahigh-speed transfer in an NRZ system has two major problems. The first problem is deterioration in waveform due to the dispersion of optical fibers have. Waveform distortion due to chromatic dispersion or polarization mode dispersion of optical fibers becomes more serious as the signal speed increases. Such waveform distortion due to chromatic dispersion or polarization mode dispersion restricts the transmission distance of an optical communication system. More specifically, the transmission distance restricted by chromatic dispersion is shortened in inverse proportion to the square of the increase in signal speed. Meanwhile, the possible transmission distance restricted by polarization mode dispersion is shortened in inverse proportion to the signal speed.
The second problem is the operational speed limit of electronic signal multiplexing circuits. For example, a 100 GHz band class electronic circuit, namely an electronic circuit capable of normal operation at a signal frequency of approximately 100 GHz is required for signal transfer at 100 Gb/s. However, it is difficult to realize such a circuit using highly economical silicon CMOS in current technology. To do so, electronic device techniques using InP materials are necessary. Even if some electronic device techniques using InP materials are applicable, various technical problems have to be overcome for stable operation because the 100 GHz class operation speed is nearly the operation speed limit of the device.
Besides the approach to increasing the operation speed of the electronic circuit for realizing ultrahigh-speed transfer, multi-level optical modulation techniques have drawn attention. Multi-level modulation is a modulation technique to assign multiple-bit information to three or more optical states. For example, the differential quadrature phase shift keying (DQPSK) has drawn attention as an important technique for 40 Gb/s optical transfer. In the DQPSK, 2-bit digital values (00, 01, 10, and 11) are assigned to four states or optical phases 0°, 90°, 180°, and 270° (“symbols” hereinafter). In this system, two bits are transferred by a symbol. Then, the symbol rate is half the bit rate. Consequently, an electronic circuit processing the symbols requires a band that is approximately half the bit rate. For example, a symbol rate of 50 G symbols/s is necessary for transferring signals at 100 Gb/s by the DQPSK.
Advantageously, an electronic circuit, such as a multiplexer and modulator driver, processing signals at 50 G symbols/s requires a band approximately up to 50 GHz. The symbol rate is lower than the bit rate; therefore, the time slot for one symbol is double the bit time slot. Consequently, waveform distortion due to dispersion is less influential and the transmission distance, which is restricted by dispersion, can advantageously be increased compared with binary modulation such as the NRZ. Using the above advantages, techniques for generating, transferring, and detecting DQPSK signal light have been under development.
Background technology relating to DQPSK signal generation is described hereinafter with reference to FIG. 11. FIG. 11 is a block diagram showing the quadrature phase shift keying unit described in FIG. 3 of Patent Literature 1. In the quadrature phase shift keying unit shown in FIG. 11, light entering an entrance end 1110 is branched into two signal lights of an equal light amount. One signal light is subject to binary, 0 or π phase, modulation in a first phase modulator 1111. The other signal light is also subject to binary, 0 or π phase, modulation in a second phase modulator 1112. After the binary modulation, a π/2 phase shift is given by a phase shifter 1113. The two phase-modulated lights are multiplexed and output from an output end 1114.
FIG. 12 presents a plane of complex coordinates showing the phases of phase-modulated lights in the quadrature phase shift keying unit shown in FIG. 11. The phase modulation effected by the first phase modulator 1111 shown in FIG. 11 is presented by phase modulation (a signal light state) 1211 in FIG. 12. Taking into account the π/2 phase shift given by the phase shifter 1113, the phase modulation effected by the second phase modulator 1112 shown in FIG. 11 is presented by phase modulation 1212 in FIG. 12. These modulated lights are multiplexed to generate a modulated light having four phase states 1213.
As described, for example, in Patent Literature 2, Mach-Zehnder modulators (Mach-Zehnder optical modulators) can be used as the two phase modulators 1111 and 1112 shown in FIG. 11 so as to more easily and precisely perform binary phase shift keying Mach-Zehnder optical modulators are often included in configurations for generating DQPSK signals.
Some methods include four-level intensity modulation in addition to the DQPSK to generate 16-level modulated signal light (for example, see Patent Literature 3). FIG. 13 is a block diagram showing the multi-level modulated signal generation part of the optical communication system described in FIG. 11 of Patent Literature 3. In the configuration shown in FIG. 13, CW (clockwise) light output from a light source 1310 is 0- or π-modulated in a first phase modulator 1311 and 0- or π/2-modulated in a second phase modulator 1312, whereby DQPSK signal light is generated by the two phase modulators 1311 and 1312. Then, the light is subject to intensity modulation in an intensity modulator 1313 driven by four-level signals. The combination of four-level phase modulation (DQPSK) and four-level intensity modulation through the process in the phase modulator 1311 and 1312 and intensity modulator 1313 yields 16-level modulated signal light. With four-level intensity modulation in addition to the DQPSK, the symbol rate is ¼ of the bit rate. Then, advantageously, for example, necessary processing speed of the symbol is only a 10 GHz class for transferring signals at 40 Gb/s. Compared with signal transfer using the DQPSK, the restricted transmission distance by polarization mode dispersion is expected to be less. In other words, the transmission distance is expected to be extended. Incidentally, the configuration shown in FIG. 11 can be used for generating DQPSK signal light.
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. H3-179939 (page 17 and FIG. 3);
Patent Literature 2: Patent Publication No. 2760856 (page 3 and FIG. 1); and
Patent Literature 3: Unexamined Japanese Patent Application KOKAI Publication No. 2006-339760 (paragraphs 0092-0095 and FIG. 11).